The present invention is directed to an improved method of forming smooth morphologies in InP-based semiconductors.
In the fabrication of nano-scale semiconductor structures, etching is critical. For nano-scale opto-electric and optical devices, surface morphology (i.e., smoothness or lack thereof) determines, at least in part, the performance characteristics of the device; smoother surfaces minimizing optical scattering losses and improving the optical performance of the device. High selectivity (i.e., the difference in etch rate between the substrate and the masking material (how deep a semiconductor may be etched without also etching away the mask)), minimal undercut (i.e., anisotropic etching), and high throughput may all be achieved with high-density-plasma etching, a technique in which an electric field causes energetic ions to strike the surface of a semiconductor substrate at an angle perpendicular to the substrate surface
Unfortunately, etching processes that produce desired characteristics (i.e., surface smoothness, verticality, etc.) for certain types of semiconductors do not necessarily produce desired characteristics for all or different semiconductors. Thus, well known and relatively mature etching processes used to etch silicon (Si) or gallium arsenide (GaAs), for example, do not yield the same results when used to etch indium phosphide (InP). This is because different semiconductors necessitate different etching chemistries to react with the substrate atoms and are thus subject to different processing issues. For example, difficulties arise in obtaining smooth-etched surfaces in InP-based semiconductors using chlorine-based chemistry etching due to the differing volatilities in the InClx and the PClx and their corresponding different removal rates during etching. Consequently, an etched surface may be either In-rich or P-rich and thus exhibit rough surface morphology. One solution has been to use methane/hydrogen chemistries, which have been shown to produce relatively smooth InP surfaces, but at very slow etch rates, e.g., approximately 100 nm/minute. Also, CH4/H2 processes lead to the deposition of polymers that can adversely alter the chamber conditions and thus affect process reproducibility.
The inductively coupled plasma reactive ion etching (ICP RIE) process is a relatively new dry etching technique developed during the 1990s. The principle advantage of ICP RIE, when compared with other RIE techniques, is the ability to independently control ion energy and flux density. That control, together with proper etch chemistry, can yield improved dry etching processes for InP-based semiconductors.
There thus exists a need in the art for a semiconductor etching process that overcomes the above-described shortcomings of the prior art.
The present invention is directed to a semiconductor dry etching process that provides deep, smooth, and vertical etching of InP-based materials with ICP RIE using a chlorinated plasma with the addition of hydrogen gas. To produce relatively high anisotropy with exceptionally smooth surfaces, the amount of hydrogen gas added preferably exceeds the volumetric measure of chlorinated gas in standard cubic centimeter per minute (sccm); at a ratio of greater than 1:1.
In an embodiment of the present invention, hydrogen is added to a chlorinated (i.e., Cl2-based, BCL3-based, SiCL4-based, etc.) ICP RIE process. The added hydrogen reacts with phosphorous to form PH3 products. To produce smooth etched surfaces in InP-based semiconductors, the amount of hydrogen gas added must exceed the volumetric measure of chlorinated gas in sccm. Preferably, the flow rate ratio of hydrogen gas to chlorinated gas is greater than 1:1, and most preferably, the ratio is approximately 5:3 when pure-Cl2 gas is used.
The inventive process provides an improved dry etching process (e.g., ICP RIE) for InP-based semiconductor materials that yields significantly improved surface smoothness (i.e., morphology), trench depths as deep as 6 xcexcm, and manufacturing etch rates of up to 1 xcexcm per minute.
In a preferred embodiment, the present invention is directed to a process for etching InP-based semiconductor materials using the ICP RIE technique. The ICP RIE system has a first RF generator connected to an inductive coil capable of transforming power into a gaseous medium to generate an ionized discharge (i.e. plasma). A second RF generator is used to create an electric field across the plasma and is connected to a platen that supports an InP semiconductor sample inside the chamber of the ICP RIE system and within which the process is carried-out. The inventive process comprises heating the platen to a temperature of between approximately 130xc2x0 and 400xc2x0 C. Reactive source gases are introduced into the chamber having a chlorine part and a hydrogen part at a higher volumetric measure (i.e., flow rate) than the chlorine part, preferably at a flow rate ratio of greater than 1:1. The pressure within the chamber is set to less than approximately 5 mT. The power of the first RF generator is set to between approximately 200 and 700 W (3000 W full scale) to provide a relatively low flux of incident ions so that a high selectivity over the etch mask (typically oxide or nitride) can be achieved. The power of the second RF generator is set to between approximately 80 and 200 W to control the DC bias ( greater than 250 V) through which the incident ions of the plasma gas are accelerated. The plasma ions are accelerated normal to the plane of the InP semiconductor surface and the substrate is etched at a rate of between approximately 0.38 and 1 xcexcm per minute.
The invention accordingly comprises the features of construction, combination of elements, and arrangement of parts which will be exemplified in the disclosure herein, and the scope of the invention will be indicated in the claims.